Porous damper for acoustical instruments



-ff@L/MM v @441 April 1, 1924, 1,488,565

G. w. STEWART ET AL POROUS DAMPER FOR ACOUSTICAL INSTRUMENTS Filed July 11 1919 v Aw 0/1/12: ///////1 x WITN E5558 ful Porous Dam Patented Apr. 1, 1924.

UNITED: STATES PATENT OFFICE.

amass w. s'mwm'r, JAMES B. nmrs'rm, Am) HOMER 1.. nonon, or IOWA crrr, IOWA.

POROUS DAMPER FOB ACOUSTICAL INSTBUIENTS.

Application and m 11, 1919. Serial in. 310,164.

To all whom it may concern: I Be it known thatwe, GEORGE W. STEWART, JAMES B. "DEMPSTER, and HOMER L. Donon, citizens ,of the United States, residing at Iowa Cit in thecounty of Johnson and State of owa, have invented a new and usep r for Acoustical Instruments, of which t efollowing is a specification.

This invention has reference to porous dampers or filters for acoustical instruments and the method ofproducing the filters.

The object of the invention is to provide means for filtering out certain disturbing characteristics of sound waves, affecting some frequencies more than others, whereby the character of the sound, or of the effects produced when sound waves are caused to set up other actions, such as the production of electrical undulations in the telephonic transmission of speech, is clarified or otherwise beneficially changed.

' In carr ing out-the invention, there is provided a til must pass, eitheratoward or from the diaphragm, whichl ter rovides minute passages' through which t e air vibrations are transmitted, and such filter provides/means whereby the filterin effect and the damping of the vibrations o the diaphragm may be controlled. J

The invention furthermore provides for the production of a porous dam er or filter of metal or other material, whereby the character, size, shape, and *other features of the pores may be made such as are needed for the effects desired.- I

The invention will be best understood from a consideration of the following detailed description, taken in connection with the accompanying drawing forming part of this s ecification, with the understanding that tie, invention is not confined to any strict conformity to the showing of the draw-- ing, or with the. method of manufaoturin the filter, but may be changed or modifie so long as such changes and modifications mark no material departure from the salient features of the invention, as expressed in the appended claims. In the drawin Figure 1 is a iametric section through a telephone transmitter, or other microphonic instrument, constructed in accordance with the invention.

ter through which the sound waves as such corrugations,

it is subjected to able material on the opposite faces of the strlip. I

V igure 6 is a fragmentary view of the strip of Figure 5, rolled upon itself into spiral form.

Referring first to Fi ures- 3- to 6, which show steps in the manu acture of a'metallic filter or porous damper, there is shown in Figure 3 a portion of a strip 1', which may be considered as a flat stri of suitable length and wider than Figure 3 being much exag erated. The strip 1 is passed throu h suitabfi rolls, or isother- Wise treated to orm one face into a series of teeth or corrugations 2, either in the form of angular teeth, or in other form producing alternate ridges and valleys. It is within the invention to produce corrugations on both opposite faces of the strip 1, but for simplicity of description, it may be considered that the corrugations 2 are produced on but one face of the stri '1. In the progress porous damper, with solder, the

the strip 1 isfirst. coated coating being indicated at thick, t e showing of of the formation of t e filter or 3 in Figure 5, and the solder-coated strip is igure 6, with the ridges or peaks of the corrugations face of the strip, thus articular structure under uralityv of passages 4, and in practice, may be 4 are likewise against the plane producing, in the consideration, a p

very minute, the passages minute and numerous.

The coiled-up strip 1 is sufliciently ex I tensive to produce a flat plate 5, shown in I face view in Figure 2-, and in cross section in Figure'l. After the stripflis coiled up as described, heat sufiicient to melt the solder coating, so that where the ridges of the corrugations engage the plane face of the strip, the solder runs together, and thus cements the strip so that it retains its coiledup shape. The melting of the solder is liable to pnoduce uneven conditions, and therefore it is desirable to machine the surfaces of the plate. For this purpose, and to prevent mashing or distortion of the passages 4, such passages are filled with parafe, or other material sufiiciently resistant to the machining operation, after which the parafiine, or other filling material, may be melted or dissolved out ofthe passages,

leaving them free and undisturbed.

The process of making the filter or porous damper permits a control ofthe number, size, shape and length of the passages 4, and a corresponding control of the effect of these passages upon sound vibrations traversing them. i

In order that the use of the filter or porous damper may be better understood, a microphonic instrument employing such structure is shown in Figure 1. The instrument includes a diaphragm 6, such as is customarily used in telephone transmitters, and the diaphragm is clamped at the edges between two rings 7, by means of screws 8, or in any other appropriate manner. The rings 7 have secured to them face plates 9 and 10, respectively, and these plates have passages 11 therethrough, produced in any suitable manner, as by making the plates skeleton plates, so that such plates will offer no sensible impediment to the free passage of air vibrations toward or from the diaphragm. The plates 9 and 10 have no damping effect, and are merely provided for supporting certain structures.

There is also provided a stem 12, having a cup 13 at one end for containing a mass 14 of carbon granules, constituting an ordinary microphonic element. carried by a boss 15, formed centrally on the plate 9,-and insulated from the boss by an insulating sleeve 17. The stem 12 is also provided with a binding post 18, for the attachment thereto of one terminal of a microphone circuit, and secured to'the plate 9, at some convenient point, is another binding post 19 for the attachment to the late or frame 90f the other terminal 0 the microphone circuit. A set screw 20, carried b the boss 15, serves to hold the microphonic element in proper relation to the diaphragm 6.

The filters or porous dam rs 5 are provided with studs 21, lodged in bosses 22, similar to the boss 15, and held in place by set screws 23. In the particular structure shown in Figure 1, a filter plate 5 is provided on each side of the diaphragm 6, and is suitably spaced therefrom. Because of the presence of the microphonic element, one of the filter plates has a central pas- The stem 12 is 1,4ae,saa

sage 24, of suflicient size to pass the cup 13, and this passage 24 is also sufficiently larger than the cup 13 to admit of the introduction of a washer 25 in surrounding relation to the cup. The washer may be of rubber, or other suitable material to practically fill and seal the passage 24 about the cup 13, so that no air vibrations can pass through. The

marginal portions of each plate 5 are each reach the diaphra m 6 through one or both of the filters 5, it ing understood that the instrument may be provided with a sin le filter 5, or with two filters, one on each side of the dia hragm, as shown in the drawing. Soun in passin to the diaphragm 6- and for simplicity 0 description it will be considered that one filter is used must traverse the porous damper oifilter 5, and

because of the minute passages through which it must travel, the sound becomes -altered in character. The effect of the filter upon certain frequencies of a complex sound will be different from its effect upon other frequencies. The filter, therefore, will damp out certain frequencies more than others, its effect being controllable by altering the size, shape, length, number, etc., of the passages or channels 4. The smaller the holes or passages, the greater is the damping of the sound. The higher the frequency of the sound vibrations, the greater is the damping. The lon r the holes or passages 4, the greater is the damping. The physical laws which govern the damping of the sound waves of high frequency without appreciable loss of those of low frequency are fully exlained and demonstrated by Lord Ray eigh on )age 332, vohune II, of his treatise entitle Theory of sound. A com lex sound, therefore, will come through sue a filter or damper reduced in intensity with the reduction greatest for the components of highest frequency. If the channels 4 are not constant in size, there is a reflection at the variations in cross section as well as absorption, but when the channels are constant in size throughout their length, absorption only occurs, wherefore channels of constant size throughout their length are the more eificient, though the invention is not necessarily confined to channels of constant size. k

The sound after passing through the porous damper or filter impinges upon the diaphragm 6, causing it to vibrate. Such tion, the air confined vibrating dia hragm tends tocontinue in vibration, wit the amplitude of the vibrations falling ofi with greater or less rapidity.

. Such falling off in amplitude must be ma ve rapid 1f the diap ragm is to follow a big 1 com lex sound or other actuatin impu se or ores, and this is the purpose 0 the damper, that is, to cause the rapid falling oil or dam ing of the vibrations of the diaphragm. I the dia hragm is in vibratween it and the porous plate or filter, is subjected to chan s in ressure of greater or less degree, epen ing among other things, upon the amplitude of vibrations. Such changes in pressure force the air out and in through the channels of the damper, retarding the motion of the air by the damping action of the damper, wherefore energy is lost, and, in turn, the motion of the diaphragm is damped. I

Not only is the damping action de ndent upon and controlled by t e size an thickness of the damper, and the size and sha of the holes, but also by the relation of t e total area of holes to the area of the damper, the distance of the dam r from the i phragm, the area covered y the damper, the presence or absence of airtight washers about the dam er, and the size of the gap in case the was ersbe absent. I

lVe are familiar with the fact that flat plates have been used for damping vibration, of a microphone diaphragm, and that these plateshave in some instances been pierced by holes. In such cases, the damping efiect is secured through the passage of air back and forth in the thin spaces bea tween the, diaphragm and plate. The motion of the air is for the most (part arallel to the surfaces of-the plate an diap ra m.

.When such plate has been pierced with ho es,

the purpose has been to allow the air to escape at other places on the late than at the edges and center. The 0 y limit as to the thinness of the plate was to retain a.

thickness necessary for mechanical strength.

To secure damping action, such a late must be ke trelatively close to the diaphragm and t e size of the channels or passages through which the air was moved changed with the motion of the diaphragm. i The actionof the filter or porous damper of the invention is radically different from such a damper plate. The loss of energy is due for the most part to the motion of air in the channels or passages through the porous damper itself. It is for this reason that 'the thickness of the damper, which controls the length of the channels, is an important factor, and the size, shape, length, total area with respect tothe area of the late, the area of the holes, etc., become actors of importance in the control of'the damping efiect. Furthermore, the distance of the late from the diaphragm ma be relative y great, and the size of the c annels or passages through which the air moves is practically independent of the motion of the dia hragm.

Anot er advanta e of the present method of damping over t at with a fiat plate is that the latter adds appreciably to the stiffness of the diaphragm, while the former does not do so to any great extent. The

herein described method of constructing the damper has the advantage of making tubular res of controllable size, length and num er, but a similar damping late could be constructed along other ines which would suggest themselves to those skilled in the art. For instance, the passages need not beperpendicular to the face of the diaphragm, nor need they be perfectlystraight and uniform in cross section, nor need metal be used. It is not necessary that the damping occur over the whole diaphragm.

As to the size'of the pores, no exact limit can be given, but they must be relatively long in comparison with their cross section.

dredths of an inch in diameter operate satlsfactorily, but they may be larger or snialler.

Pores 'of two hundredths or three hun- When a damper is used behind the diahra , it will, of course, not be necessary or t e pores to extend all the way through,

but to extend onl for a sufiicient depth to absorb the high requency waves, whlle the low frequenc with slight ecrease in intensit It is also apparentthat thefllters or porous dam rs may be otherwise arran ed than in t e showing of Figure 1, which waves will be reflected backshowing is to be taken more as illustrative of one of many embodiments of the invention than as confinin the invention to the particular showing 0 Figure 1.

Moreover, the invention may be used in connection with the reception .and trans-' consisting of a strip of metal" spirally wound into a Hat plate, adjacent convolu-' tions of said spiral being suitably held in rigid contact, one ed ofsaid strip being ag, said' corrugations milled or corru I forming numerous pores through the plate. v2. A damper for an acoustic instrument into a flat plate, adjacent convolutions of said s iral being suitablgecemented together, one ge of said strip ing milled or corcomprising a strip of metal spirally wound rugated so as to form numerous pores throu h the plate.

' 3. damper for an acoustic instrument comprising a strip of metal spirally wound into a flat plate, adjacent oonvolutions of said spiral being suitably held in rigid contact, one edge of said strip being traversed by numerous minute parallel corrugations of uniform size to form pores through the plate of comparatively small cross sectional area.

4. In an acoustic instrument, the combination with a vibratory diaphragm, of a damper plate associated therewith and provided with minute pores or channels of a diameter and length to absorb or dissipate the major portion of the high itch components of sound waves passing therethrough and to let through the major portion of the low pitch components of such sound waves.

5,, A damper for an acoustic instrument comprising afiat plate formed by juxta posing plane and corrugated surfaces to form pores or channels extending from face to face of said plate:

6. In an acoustic instrument, the combination with a vibratory diaphragm of porous materiai, adapted to be interposed in the path or" sound waves approaching or receding therefrom, of a damper plate having tubular ores of substantiall constant and relative y small cross sectional area throughout their length and extending substantially erpendicular to the diaphragm, whereby t e character of the sound wave with respect to the distribution of energy among t is changed. v

b7. The cpmbfiiiiation wc'lith a vibll'ator di}are 0 a teror am rpateintie ath d f sound directed tdi and or away rom the diaphragm, said late being provided with a multiplicity of minute tubular pores or channels therethrough, said pores ing relatively long in comparison with their diameter, whereby dissipation of energy is secured by the flow of the Vibrating fiulid in and through said pores or channe s.-

In testimony, that we claim the foregoing as our own, we have hereto afiixed our signatures.

GEORGE WV STEWART. JAMES B. DEMPSTER. HOMER L, DODGE Witnesses:

L. W. W. Momzow, HAROLD N. BOZELL.

e different frequencies or pitches 

